Soundstream USA204

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Hello: I just finished repairing my USA204 amp by replacing the MoSFETs and the PWM (SG3524) chip in the SMPS section. The original design uses the BUZ11 but since I have about eight RFP50N06 lying around in my parts bin, I used them instead since they have a much higher rating. They differ in their internal drain to source diode. The RFP50N06 uses a zener diode. The problem now is that the turn-on delay is about three minutes:eek: . After checking the schematic (courtesy of Sounstream Technologies), I probed pin16. when power is applied this pin starts out at about 10V and gradually decreases to about 1.3V, then the amp turns on. I replaced C8 (100uF/16V) with a 1uF/16 and R9 (22.1K) with 150K. Now the delay is about eight seconds. I tested the value of the original resistor and it is 22K exactly. I tested the amp in bridged mode for about half a day and no problems with the sound although the chassis was quite hot. Now when I apply power the power-on LED slowly lights up to full brightness instead of an instant-on condition after the delay. Can anyone give an explanation for this turn-on delay behavior?
 
I think the 3524 may have seen a production change. I've seen a couple of amps that required a 10k ohm resistor between pin 8 and 10 to return the delay to something near the intended delay. If I'm not mistaken, I didn't change the delay cap.

The power supply FETs should not get as hot as the output transistors (generally BJTs in the soundstream amps I've seen). If the power supply transistors are getting hotter than the output transistors, there's likely a problem with the drive circuit for the power supply transistors.
 
The 10% tolerance resistor may be exactly 75 ohms. You should check it to confirm that it's not too far off. If you have several, check them all and use the one that's the closest to 75 ohms.

As was suggested, it's best to use the original part number as replacements. You always want to reduce the number of variables when troubleshooting a circuit.

Can you measure the operating frequency of the power supply? Sometimes you can have a shift in value of the timing components (resistor or capacitor) and the operating frequency will be too high or too low.
 
I just opened the amp again today to take some measurements and I found out that the input filter capacitors were blown (C2,C3). I replaced them and measured the idle current (no input signal and no load speakers connected) to start out at about 7Amps, gradually settling down to about 6.3Amps. The input filter capacitors then were getting warmer and warmer. Attached are photos of the waveforms.
 

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You are correct. My approximate calculation is about 56kHz. If the frequency is higher does it follow that the output voltage will also increase or will the duty cycle compensate for the frequency increase to keep the output voltage at the set value? Could this also be the reason for the rise in temperature and subsequent blowing of the caps?
 
An increase in frequency won't necessarily change the output voltage.

What could be happening is the FETs are not being turned off quickly enough. If this is the case, you'd have both banks of FETs on for a short period of each cycle. This could lead to excessive current draw and excess ripple current for the input filter caps. This could be the reason they're heating up.

If you monitor the voltage on the gate side of the gate resistor, you'll likely see that one bank begins to turn on before the other turns off (voltage falls below ~3.3 volts). If the gate waveforms from opposite banks are both above ~3.3 volts at any point in time, it's a problem. Generally, you'd want to see the gate voltage drop to below 1 volt on one bank before the other bank begins to turn on.
 
I checked the waveforms directly at the gate terminal and they were the same as when checked on pins 11 and 14. By the way the actual waveforms were in-phase, I just inverted the other channel to view them symmetrically:confused: . Aren't they supposed to be out-of-phase? The datasheet of the SG3524 gave the formula for the frequency as f=1.18/RtCt. With the values in the schematic (Rt=1.9k and Ct=.0047uF), f=132kHz. In push-pull configuration it is effectively about 66kHz.

I will try to remove the gate drive from one bank of FETs and see what happens.
 
Don't try to run the amp with only one bank connected. The other bank will likely fail.

The outputs have to be out of phase. Double-check your setup. Place both probes on the same point. They should give precisely the same trace.

The waveform should have been slightly different on the other side of the gate resistor but with 10 ohm gate resistors, it may be difficult to see the difference.

I have an SA245 here and it has a similar supply. The attached image shows the waveforms on the gates. The timebase is at 5uS. The vertical amp is at 5v/div.

Can you post a photo of the waveforms on the primary windings?

This amp has a 1.91k ohm timing resistor and a 4.7nF timing cap. For some reason, it's oscillating at 53.35kHz instead of the calculated frequency of 66kHz.

I also noticed that the amplitude of the traces is different for your amp. Is that a calibration error or is it the true amplitude?
 

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I see. I just ran the amp without the drive to one bank of the FETs for about two minutes:eek: . The amp did turn on and the filter caps did not get hot to the touch although they warmed up a bit. Placing both probes on the same point gave precisely the same trace. The different amplitudes of my traces could be accounted for the vertical position setting or a calibration error. I'll run the test again with another scope to check the amplitudes and also post some photos of the primary winding waveforms. We do have the same values for Rt and Ct. Thanks for your help so far. I've learned a lot from this discussion:) .
 
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